Carbon nanotubes, once the objects only of research and characterization, are finding increasing usefulness in a variety of applications. They are being employed, for example, as components of fibers for inclusion in weaving rugs that will then have antistatic properties, as components of moldable resins, and as components of films used as static-free packaging for electronics. See, e.g., U.S. Pat. No. 6,426,134. Carbon nanotubes have also been taught as being useful in the piezoelectric generation of electrical energy, according to U.S. Pat. No. 6,559,550. Carbon nanotubes are used as absorbents or as the tips of probes used in various scanning microscopes, and many additional uses are known and being found.
There are several methods of creating carbon nanotubes. A common method is arcing or sputtering. A current is passed between two separated electrodes, either or both of which are graphite or another carbon compound. What is formed is a mass, sometimes called a boule, of carbon material. Some of the mass consists of the carbon nanotubes, while the remainder consists of amorphous carbon, other fullerenes, graphite, and the like.
Other methods of producing carbon nanotubes have been proposed. Laser ablation of a carbon surface is one and catalytic vapor deposition is another. It has also been proposed to create nanotubes by heating a SiC crystal under a vacuum, whereby the silicon atoms are removed from the crystal leaving behind oriented carbon nanotubes.
While some of these methods, such as that disclosed in U.S. Pat. No. 6,303,094 relating to the above-referenced SiC crystal method, are said to produce very high purity nanotubes, most methods do not. The process of forming the carbon nanotubes almost invariably results in the formation of the other carbon substances such as amorphous carbon and, depending on the process, other carbon or non-carbon substances may be mixed in with the carbon nanotubes. For the purposes of this application, all substances, whether carbon or carbon compounds or non-carbon materials, other than the nanotubes themselves, are referred to herein as “soot.”
A problem to be addressed before the nanotubes can be utilized, then, is separating the nanotubes from the soot or, put another way, to purify the nanotubes. A method of purification mentioned in U.S. Pat. No. 6,422,450 is to suspend an as-produced mixture of nanotubes and soot in an alcohol by subjecting the mixture to a high-power ultrasonic horn, and then filtering the suspension through a micro-port membrane. It is disclosed that the use of the horn may serve to damage or create defects in the nanotubes. Another method, mentioned in U.S. Pat. No. 6,426,134, requires refluxing impure nanotubes in 2.6 M nitric acid, then resuspending the mixture in pH 10 water with a surfactant such as sodium lauryl sulfate, followed by filtration in a cross-flow filtration system and filtration through a polytetrafluoroethylene filter.
The first method cited above requires the use of special equipment, and damages the nanotubes by removing the ends and creating other defects. The second method requires the use of harsh reactants, which may also damage the nanotubes, and requires several steps. In addition, this second method results in the oxidation of at least the ends of the nanotubes.
For many purposes in both research and commercial applications it is desired to have a plentiful source of undamaged nanotubes. Such nanotubes are sometimes referred to as native state nanotubes because, other than the purification, they have not been chemically or mechanically altered after being created by whatever method was utilized to create the nanotubes. Such native state nanotubes may have, whether by intent or not, certain defects in structure. The nanotubes may also, again whether by intent or not, chemically or mechanically contain atoms or molecules other than carbon. In any case, what is needed in the art is a simple method of purifying carbon nanotubes without damaging the nanotubes and without using reagents that are harmful to the nanotubes, the person practicing the method, or the environment.